Lenses comprised of high refractive index and low dispersion glass permit compact optical systems with chromatic aberration correction when combined with lenses comprised of ultra-low dispersion glass. Thus, they occupy an extremely important position as optical elements constituting image pickup optical systems and projection optical systems such as projectors.
The optical glass (glass type TAFD25 (refractive index nd 1.90366, Abbé number vd 31.32)) made by Hoya Corp. that is described in OPTICAL GLASS Technical Data 2011 (released by Hoya Corp.) is an example of such high refractive index and low dispersion glass.
With the heightened specifications and high functionality of lenses in recent years, the importance of the partial dispersion characteristics of optical glasses that are lens materials has increased. Glasses that are particularly desirable for chromatic aberration correction have high refractive indexes and low dispersion, as well as being optical glass with low deviation ΔPg,F (abnormal dispersion) from the normal line of the partial dispersion ratio Pg,F.
The above optical glass TAFD25 is an extremely good glass material because it is high refractive index and low dispersion glass with extremely low coloration. However, the ΔPg,F is 0.0028; optical glass with lower ΔPg,F would be desirable for the correction of high-order chromatic aberration.
High refractive index and low dispersion glass is also disclosed in Japanese Unexamined Patent Publication (KOKAI) Showa No. 60-131845 or English language family members U.S. Pat. No. 4,584,279 and US2011/0028300A1, Japanese Unexamined Patent Publication (KOKAI) No. 2010-083705 or English language family members US2012/0142517A1 and U.S. Pat. No. 8,127,570, which are expressly incorporated herein by reference in their entirety.